i've only done 1 tiered cake, and i covered the whole board with foil. I was just unsure how the board was handled (hygienically) before i got hold of them, so .....

just like i always wash my cake bases before putting my cakes on them to be iced. i'm sure they must, at one point, been lying on the floor somewhere, along the way from the factory to the retail cake outlets.

in the future, if i were to decorate tiered cakes again, i will make sure i cover the boards.

Never had issues and I don't cover mine. Sure it absorbs some oil but not enough to be soggy. I have been known to glue two cakeboards together though for tiered cakes. Now I'm switching to SPS so the plastic plates will prevent any bending with a cardboard plate. I guess it just depends what method works for you.